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Nontraumatic inflammatory joint disease is a relatively common, but under-recognized, cause of fever and morbidity in dogs. In one review of 66 dogs referred for fever of unknown origin to a veterinary teaching hospital, approximately 8% were diagnosed with immune-mediated polyarthritis. In addition to immune-mediated polyarthritis (i.e. inflammatory joint disease in the absence of an underlying genetic, infectious, or other secondary cause) there are a number of additional syndromes that have been recognized in dogs and cats. This lecture will review the general approach to non-traumatic joint disease in small animal patients, the differential diagnoses encountered in dogs and cats, and then review the clinical features and treatment of immune-mediated polyarthritis in detail.

DEFINITION AND DIFFERENTIAL DIAGNOSIS

Polyarthritis is defined as inflammation in two or more joints. Although any joint may be affected, including the inter-vertebral joints, large proximal limb joints and small distal limb joints, it is the stifle, carpal, and tarsal joints which are usually most severely affected. In addition, swelling due to effusion is easiest to appreciate in these three sets of joints. Primary immune mediated polyarthritis is a type III hypersensitivity disorder—circulating immune complexes are deposited in the synovial membrane and initiate inflammation, recruitment of inflammatory cells, release of cytokines and proteolytic enzymes including metalloproteinases, with resultant pain, excess joint fluid production, and eventual cartilage degeneration. In secondary polyarthritis, immune complex deposition in the joints may occur secondary to any concurrent inflammatory/infectious or neoplastic disease. Some of the best recognized causes of secondary immune-mediated polyarthritis are the chronic tick-borne infectious agents, including Anaplasma spp, Ehrlichia spp, and Borreliaburgdorferi, in dogs, and a post-calicivirus vaccine transient polyarthritis that may occasionally be seen in cats.

The forms of immune-mediated polyarthritis described in dogs and cats include: